1. Field of the Invention
The present invention relates to a luminous flux separating optical system and, in particular, to a color separation prism which can be used in a TV camera and the like.
2. Description of the Related Art
An image luminous flux formed by a lens is often separated and image processed for various purposes. It is conventionally known that, as means for separating such luminous flux, there has been used a luminous flux separating optical system which is composed of a plurality of prisms. For example, in a color TV camera, the image luminous flux is image-processed by separating the luminous flux into a RED component, a GREEN component and a BLUE component. For such color separation, as shown in FIG. 1, a prism 101 is covered on the luminous flux leaving surface 12, with a dichroic coating which can reflect only the BLUE component, and the reflected light of the BLUE component is totally reflected by a surface 11 and is then image-formed on a BLUE image pick-up element 15. The remaining luminous components are allowed to pass through an air gap 18 and, after then, only the RED component is reflected by a dichroic coating which is coated on a surface 14 and is able to reflect the RED component. The reflected RED component is totally reflected due to a difference between the indices of refraction of the air gap and a prism glass and is then image-formed on a RED image pick-up element 16. The remaining luminous flux component is the GREEN component, which is image-formed on a GREEN image pick-up element 17. This is a typical conventional method of separating a luminous flux. There are other known luminous flux optical systems according to the prior art. These systems are different from the above-mentioned typical system in the characteristics of dichroic coatings and the number of separation, but they are similar to the typical conventional method in that, in order for the separated luminous flux components to be reflected totally, prisms are provided in the optical system with the mutually facing surfaces thereof disposed in parallel to each other and at a certain distance from each other, and there is formed a parallel-shaped air gap having an angle of inclination, which is not a right angle with respect to the optical axis of the system.
In general, it is known that an optical discontinuous surface not perpendicular to the optical axis acts to produce an astigmatism with respect to an image luminous flux which passes through the optical discontinuous surface. For this reason, the above-mentioned parallel air gap, which is an optical discontinuous surface in the luminous flux separating optical system, produces the astigmatism which can deteriorate the performance of the separating optical system. This can be recognized quantitatively as follows:
For the purpose of simplification of the problem, let us consider an optical system having one parallel air gap as shown in FIG. 2. If a dichroic coating and the like used for color separation are omitted, then the above-mentioned prisms can be considered as two simple prisms which have an air gap 28 between them, as shown in FIG. 2. By ray tracing a paraxial astigmatism in the optical system, it is possible to calculate easily an astigmatism .DELTA..sub.0 which is caused by the parallel-shaped air gap 28. That is: ##EQU1## where .DELTA..sub.0 : difference (t-s) between T image surface image forming position and S image surface forming position, that is, astigmatism
d: thickness of air gap 28 PA1 n: index of refractive index refraction of prism glass PA1 1/4: angle between air gap and surface perpendicular to optical axis.
Such astigmatism .DELTA..sub.0 deteriorates the performance of the optical system.